Allow contributors to use Zeus for fast Rails commands & tests

[Zeus](https://github.com/burke/zeus) is a great gem that preloads
your Rails app so you can run commands and tests quickly. However,
it doesn't play well with the default "spec_helper.rb" that gets
generated when you install Rspec. The solution is to remove
"require 'rspec/autorun'" from "spec_helper.rb", which is what this
commit does.

This change does not affect those who do not have Zeus installed.
Running "rspec" from the command line will continue to work.

Reference and more info:
https://github.com/burke/zeus/issues/180
http://railscasts.com/episodes/412-fast-rails-commands
http://robots.thoughtbot.com/post/40193452558/improving-rails-boot-time-with-zeus